1. Field of the Invention
The present invention relates to an image forming apparatus employing an LED (light emitting diode) printing head. More particularly, the present invention relates to an image forming apparatus such as a digital electrophotographic copying machine, a printer, etc.
2. Description of the Prior Art
Digital image forming apparatuses have conventionally been known which have an LED printing head as writing means. Generally, digital image forming apparatuses are provided with a photoreceptor, charging apparatus for forming a charging portion on the photoreceptor, an LED printing head for forming a latent image by irradiating light on the charging portion of the photoreceptor based on image information and a developing apparatus for developing the latent image formed on the photoreceptor into a toner image. A plurality of LEDs are provided in an array on the LED printing head. An exposure energy required for a formation of a latent image is determined by an emission energy and emission duration of the LEDs.
For example, when the toner development is a reversal development where toner adheres to a latent image on a portion on which light is irradiated by the LED printing head, if an original has a lot of black portions or if a copying operation is continuously performed (for example, a continuous copying is performed), the LEDs generate heat since an activation duration for the LEDs increases. An emission energy of the LED which is generally used decreases as temperature increases. Therefore, when the activation duration for the LED increases according to image information, there arises a problem that the exposure energy decreases to reduce an image density so that an image quality deteriorates.
To overcome such a problem, Japanese laid-open Patent Application No. S61-255366 discloses a feature where a temperature of the LED printing head is detected by a temperature sensor and a duty ratio of a light quantity signal of the LED is changed according to a change in the temperature.
Specifically, the duty ratio of the light quantity control signal is controlled so as to increase when a temperature of the LED printing head increases. In this method, however, the activation duration for the LED increases to further increase a temperature of the LED printing head, so that the image quality inevitably deteriorates.